1. Field of the Invention
The present invention relates generally to a composition used to attach electronic components to a printed wiring board. More particularly, the present invention relates to urethane compositions which have been modified by the addition of an epoxy component and which are able to flow under pre-installed components on printed wiring boards.
2. Description of Related Art
In the fabrication of electronic devices, the individual electronic components are attached to the printed wiring board by the use of adhesive. Typical procedures consist of dispensing the adhesive onto the surface of the printed wiring board, installing the component in the desired location, curing the adhesive, soldering the leads from the component to the board, and removing the solder flux. The previously used adhesives required curing prior to the soldering operation because the uncured adhesive could not withstand the flux removal process. In this prior procedure, the position of the component relative to the printed wiring board sometimes changed during the heating step required for the curing of the adhesive. Consequently, lead bends required adjustment after the adhesive was cured in order to compensate for this shift. However, some degree of stress is induced in the leads, and complete contact of the adhesive with the underside of the component may be compromised.
Many different adhesives have been developed for attaching electronic components to printed wiring boards. Polyurethane resins have been used because of their high insulation resistance, good flexibility and low glass transition temperature. However, polyurethanes typically have a relatively short work life and storage life as a frozen premex, which makes them unacceptable for use in a mass production environment. In addition, polyurethanes do not exhibit good high temperature stability.
Another material which is widely used in the electronics industry as an adhesive consists of epoxy resin. Epoxy compounds typically have good insulating properties and excellent adhesion, and are easy to process. However, some epoxies may exhibit poor hydrolytic stability and mechanical properties, such as excessively high modulus and poor removability.
Thus, a need exists in the electronics industry for an adhesive composition which can minimize lead stress and provide uniform contact with the component, while at the same time possessing good processing characteristics and good physical properties when cured.